Watch mainspring



Patented a. 3, 1950 WATCH MAINSPRING Oscar E. Harder and Dimon A. Roberts, Columbus, Ohio, assignors, by mesne assignments, to, Elgin National Watch Company, Elgin, Ill., a corporation of Illinois No Drawing. Application May 3, 1947,

Serial No. 745,714

4 Claims. (Cl. 267-1) .This invention relates to power springs, oi which the mainspring of a watch is an example of practice presenting extreme problems in that space and probable life considerations in a watch demand the capability of being able to store a high amount of energy in a small space with assurance that repeated winding and unwinding during th life of the watch will represent the storage and release of substantially identical amounts of energy.

This application is a-continuation-in-part of our copending application Serial No. 567,894, filed December 12, 1944, now abandoned.

A characteristic of such power springs is that, during their use, they are under continuous loaded conditions and, by design for economy, the load at the end of winding is close to the strength limits of the material, the least feasible factor of safety being allowed.

Prior to our invention, watch mainsprings have been customarily made of high-carbon spring steel as this is the only material which has been found to have the necessary combinations of high strength properties and toughness to store the required amounts of energy in the small space available. The spring steel currently used in watch mainsprings has a yield strength of around 232,000 p. s. i., and a modulus of elasticity of around 28,400,000 p. s. 1.

Two defects of steel watch mainsprings have long been apparent. One of these is a tendency to corrosion in the presence of moisture. As watch springs are stressed near their ultimate of strength in use, only slight corrosion causes them to break.

A second defect is the tendency to take a permanent set" which shortens the eifective length of the springs and decreases the amount of energy which can be put into them. This defect results from the fact that spring steel has a proportional limit well below its yield strength. The proportional limit of ordinary watch spring steel is only about 177,000 p. s. 1., so that any stressing over this amount leads to apermanent set and, at the least, has the eifects of shortening the length of time the watch will run for a single complete winding, andof altering its accuracy due to reduction of average torque delivered to the train while unwinding and driving.

We have succeeded in overcoming both of these defects by providing a watch mainspring of a non-corrosive material which has a yield strength and modulus of elasticity as great as those of spring steel and has a proportional limit well above that of spring steel. Our new watch mainspring is made of a non-corrosive alloy which is solution-annealed to give it the relative softness and other properties necessary to permit coldworking. A yield strength and elasticity equal to those of spring-steel and a porportional limit remarkably higher than that of spring steel are developed in this material by elongation by coldworking followed by heat-aging. The combination of these two steps has the effect of increasing the hardness and strength of the material and at the same time the effect of causing an increase of as much as 75 percent in the proportional limit of the material.

The alloy which is subjected to this treatment to produce the new spring consists principally of cobalt which gives it strength, and chromium which gives it strength and corrosion resistance, and a plasticizer including nickel which makes the alloy when in solution-annealed condition sufliciently plastic to be susceptible of a high percentage of elongation by cold-working. The solution-annealed alloy includes precipitatable components which during aging develop hardness and I in greatly increasing its tensile strength and proportional limit. The strength inherent in the matrix of cobalt and chromium, coupled with the increase ofstrength resulting from the cold elongation followed by aging, lead to the development of tensile strength as great as that of watch mainspring steel and a proportional limit materially greater than that of watch mainspring In the above andother tables and data, TS represents tensile strength, PL proportional limit, YS yield strength, Mod modulus of elasticity, and VHN the Vickers Hardness Number. TS, PL and Y8 are in thousands of pounds per square inch (p. s. i.) and Mod is in millions p. s. 1. Y8 is given with 0.02 percent oflset. Bend tests, which are an index of toughness, were 180 around the arbors shown.

The tapes were then subjected to aging at 900 degrees F. for 5 hours, and, upon test, had the following properties:

Table IV Tensile Properties Bend Test Diameter, Inches All Part All TS PL Ys Mo 1 VHN Bend Break Break Nora-Alloy No.- 12 is of low strength and hardness. Alloys Nos. 14 and 15 were too brittle to test.

The alloy composition basically contains about 20 to 50 percent of cobalt and about 15 to 30 percent chromium, along with 20 to 50 percent of softening-or plasticizing component including nickel. In general, these components are present as 20 to 50 percent of cobalt, 20 to 37 percent of chromium and molybdenum combined (the molybdenum being 1 to percent of the alloy), and 20 to 50 percent of nickel, iron and manganese combined (the percentage of nickel being greater than that of the-iron, with negligible traces to a maximum of percent of iron, and the percentage of manganese being from a residual fraction to 5), and with a carbon content of about 0.05 to 0.30 percent. Beryllium may be present in low amounts, specifically from 0.01 to 0.09 percent having been found of benefit for watch mainsprings. The remainder consists of impurities concurrent with the metals introduced and those resulting from melting procedures. They should not comprise in excess of 0.05 percent of sulphur, 0.05 percent of phosphorus,

and 0.05 percent of nitrogen (the nitrogen being effective as a partial substitute for carbon). Residual silicon from deoxidation during melting can be tolerated up to 0.50 percent, and in some alloys up to 1 percent when the iron content is high. Thus, the total of such concurrent and denum), and 25.5 to 40.3 percent of nickel, iron and manganese combined (of which the iron is less than the nickel, and about 0.5 to 2 percent is manganese). For the remainder, the alloy has about 0.08 percent to about 0.22 percent of carbon; 0 to 0.09 percent beryllium; less than about 0.15 to 0.25 percent silicon being preferred; less than 0.05 percent each of sulphur and phosphorus; other elements being present in nonsigniflcant traces. The total of such concurrent and residual elements is less than 0.5 percent.

The cobalt is a matrix ingredient to give strength. In general, increase of strength follows with increase of cobalt, but excessive amounts produce such hardness that cold-working characteristics are unsatisfactory, as the desired final strengths cannot be built up. The cobalt is furthermore believed to form an intermetallic compound with chromium which provides a hardening and strengthening component during aging.

The chromium contributes most importantly to the corrosion resistance, and cooperates with the molybdenum in that increase of one or theother or both, above the lowest values stated, lead to increased strength and hardness, so that the sum of the chromium and molybdenum is determinative when both are present.

The molybdenum is a very effective strengthening element both for its effect in the matrix and upon aging.

The plasticizing metals 'of the group consisting 1* of nickel, iron and manganese are regarded as softeners of the composition in solution-annealed condition. That is, a binary alloy consisting *of g cobalt and chromium alone, but having good ratios of cobalt and chromium for development of strength upon cold working and aging, in accordance with our studies, is not cold workable to suflicient extent to build up its strength to the superior values which can be attained with our alloy; but upon addition of such plasticizers, the hardness of the solution-annealed alloy is reduced so that cold-worked strengths can be developed before the hardness has been increased by the cold-rolling to values which render further coldworking impracticable. In general, nickel by itself is eifective; and it can be used without significant amounts of iron or manganese. In practice, iron can be used as a minor replacement for nickel and also with the great saving in that the chromium, molybdenum and manganese can be introduced as the corresponding ferro alloys which are cheaper per weight of chromium or molybdenum and also have lowermelting points and thus facilitate the smelting. Iron is not permissible as a total replacement, however, due to scaling trouble upon heating;

and its amount should be kept below that of the,

nickel. Manganese is a good deoxidizer during mixing, and also acts to overcome any harmful. effect of sulfur: in the final alloy, the residual manganese cooperates with the nickel in giving. the softness or workability desired: up to 5 percent can be present without harmful effect, but no specific advantage appears with more than about 2 percent.

The preference for carbon contents of 0.08 to 0.22 percent has been indicated by the above satisfactory compositions. The effect of carbon is illustrated by otherwise identical alloys of Type 3, of which Alloy A had 0.05 percent and Alloy B had 0.09 percent carbon as shown in the following Table V. The cold rolled condition is that given by solution-annealing and cold-rolling, while the aged" condition is samefollowed by aging for 5 hours at 900 degrees F.

Thus, a most significant improvement in proportional limit is attained, being the property of a power spring which determines whether or not the spring will set in servic and how much it will set.

The term solution-annealing," as employed herein, defines the operation of heating the mass to a temperature at which apparent homogenization occurs, that is, at which precipitation components are brought into solution; and the cooling of the homogenized mass in order to fix this condition. This cooling must be rapid enough to prevent precipitation of such components, or premature aging, as such would lead to an undesirable hardness and resistance to cold working. In practice, the alloy should be given an initial solution-annealing at and from a temperature of 2000 to 2300 degrees F., and preferably from 2100 to 2150 degrees F.; and thereafter the intermediate and final solution-annealings can be conducted at and from temperatures as low as 1800 degrees F., but preferably from 2000 to 2100 degrees F. The rapid cooling of sections thicker than 0.200 inch requires waterquenching, while thinner sections can be effectively and more conveniently cooled in air.

The effect of the heating is to soften the alloy to suitable condition for cold working, to put certain secondary constituents into solution and to tend to produce a homogeneous structure having a faced-centered cubic arrangement, and to put the alloy into condition for good response to the age-hardening treatment.

The effect of the temperature during solutionannealing is illustrated by Alloy No. 3, which before cold-rolling had a Vickers Hardness of 240 and which was then reduced 50 percent by coldrolling, and was then quench-annealed.

Table VI Hardl i P Time ness Control as cold-rolled, 468

50 percrnt reduction.

minutes. 550 30 minutes. 485

l, 400 30 minutes. 455 1, 500 30 minutes. 412 l, 600 30 minutes. 343 l, 800 30 minutes. 302 2, 100 30 minutes. 240

- in order to minimize scaling and roughening the surface of the stock.

The hardness at the beginning of final coldrolling should be below 300 Vickers (not Table II) and alloys as low as 200 Vickers have been found competent of attaining final strengths of satisfactory values. The hardness and strength increase quite rapidly in the first stages of coldreduction and then less rapidly. For exampde, Alloy No. 3 at reductions of 75, 80, and percent showed hardness values of about 510, 570, 580 and 590 respectively. For watch mainsprings the cold-reduction should be carried as far as practicable, and the hardness should be at least 450 Vickers (note Table III). The thickness reduction for such mainsprings should be at least about '70 percent, and it is preferred to have reductions of at least 80 percent, and reductions in excess of 90 percent have been found of value.

The aging treatment has as its principal function the increasing of the proportional limit, the yield strength, the ultimate strength, and the modulus of elasticity. The response in the aging temperature depends to some extent upon the composition of the alloy; and is a function of the amount of the cold reduction, the final thickness of the article, the aging time, and the aging temperature.

The temperature for aging our alloy is 500 to 1200 F. The present commercial practice of aging is heating to 700 to l F. for times, dependent upon the temperature used. A time of 5 hours at 900 F. has been found to give a desirable combination of strength and toughness properties. In general, the proportional limit, yield strength, and tensile strength increase with the aging temperature, up to temperatures of about 900 to 1000 F., but this increase in strength properties is at least to some extent accompanied by decrease in toughness. In particular, overaging must be avoided because heating for such a period of time as 5 hours at temperatures of above 1200 F. causes a marked decrease in the strength properties with inadequate, if any, compensation in ductility. It may be theorized that a condition of solid solution supersaturated in precipitatable components, produced by solutionannealing, is modified by aging, in that the precipitation particles come out in submicroscopic size and in total amount corresponding to the difference in solubilities at solution temperature and at aging temperature, and when the supersaturation has essentially ceased by formation of the submicroscopic particles, a maximum of strength properties has been attained; while higher temperatures will cause more rapid precipitation but also more rapid agglomeration. In practice, 1200 degrees F. is the maximum useful temperature: but it is preferred to use lower temperatures, for the reason that the time factor is then not so critical.

In general, lengthy exposure at, temperatures between 1200 and 1800 degrees F. should be avoided after the final solution-annealing. If the material is slowly dropping in temperature from solution-annealing conditions, premature precipitation with agglomeration occurs so that the material becomes too hard for cold-working and incapable of developing the strength values which can be induced by proper cold-rolling and aging; if the material is slowly increasing in temperature within this range, agglomeration occurs and then re-solution, but the homogenization does not proceed to the necessary reduction of hardness for proper cold-working or to the condition for effective precipitation during aging.

The article thus formed and constituted is a power spring capable of withstanding service conditions at which a spring of high-carbon steel, but of identical size and use, will fail. It demonstrates a tensile strength and proportional limit exceeding those of the steel spring, and under preferred conditions (Table IV) has a proportional limit in excess of about 200,000 p. s. i.; a yield strength in excess of about 250,000 p. s. i.; and a modulus of elasticity of above about 1 29,000,000 p. s. i.; as compared with high-quality carbon steel mainsprings which may have a proportional limit of 177,000 p. s. i.; a yield strength around 232,000 p. s. i. and a modulus of elasticity around 28,400,000 p. s. i. It is non-corrosive to atmospheric conditions, to perspiration, and is highly resistant even to strong acid and alkaline solutions. It is essentially non-magnetic and non-magnetizable. The eifects of the procedure of preparation are apparent in the article: in that such a spring, when heated to 2100 degrees F. and quickly cooled undergoes severe loss of strength and hardness; in particular, the hardness will drop to below 300 Vickers, and the tensile strength falls below 200,000 pQs. i., and the article is no longer competent of use, for the reason that no known method will restore the desired values without a high cold-reduction as stated above, that is, without reducing its section and increasing its length so that it no longer is the same article. In this respect, the article is strikingly different from a carbon steel spring, which can be repeatedly hardened and tempered if care. be taken to avoid scaling and decarbonization.

The practical limit of cold working our alloy, as by cold rolling, is fixed by that degree of cold working which causes damage to the material such as excessive edge cracking and surface cracking; by the degree of cold working beyond which further cold working gives little or no improvement in. the strength properties of the material as cold worked or in the enhancement of the strength properties on aging; and also to some extent by the plant equipment available, but the cold working should give at least the minimum reduction disclosed hereinbefore in this specification.

The process referred to herein is set out and claimed in our concurrently filed application, Serial No. 745,715, and certain of the present alloys are set out and claimed in our concurrently filed application, Serial No. 745,716.

We claim:

'1. A watch mainspring characterized in being non-magnetic, non-magnetizable and non-corrodible by atmospheric conditions and body fluids,

'said mainspring comprising a metallic ribbon having beer'i sd'liition-annealed, cold-rolled and age-hardened in sequence, and so conditioned having a hardness above 480 Vickers, a proportional limit of at least 190,000 p. s. i., a tensile strength of at least 316,000 p. s. i., a yield strength (0.02 per cent offset) of at least 225,000 p. s. i., and a modulus of elasticity of at least 28,400,000 p. s. i., said ribbon being of an alloy consisting essentially of to 50 per cent cobalt, 20 to 37 per cent chromium and molybdenum together, and of which 15 to per cent is chromium and l to 10 per cent is molybdenum, 20. to 50 per cent of nickel, iron, and manganese, the amount of nickel being greater than the amount of iron. the amount of iron being not more than 15 per cent, and the amount of manganese being less than 5 per cent, and from 0.05'to 0.30 per cent of carbon, the composition of the ranges being 5 melted alloy, the said alloy having a hardness of from 200 to 300 Vickers in solution-anealed condition and being capable in solution-annealed condition of reduction by cold-rolling by at least per cent.

l 2. A watch mainspring characterized in being non-magnetic, non-magnetizable and non-corrodible by atmospheric conditions and body fluids, said mainspring comprising a metallic ribbon having been solution-annealed, cold-rolled and age-hardened in sequence, and so conditiond having a hardness above 575 Vickers, a proportional limit of at least 200,000 p. s. i., a yield strength (0.02 per cent offset) of at least 250,000 p. s. i., and a modulus of elasticity of at least 28,400,000 p. s. i., said ribbon being of an alloy consisting essentially of 28 to 45 per cent cobalt, 24 to 35 per cent chromium and molybdenum together, of which 5 to 7 per cent is molybdenum, 25.5 to 40.3 per cent of nickel, iron, and manganese, the amount of nickel being greater than the amount of iron, the amount of iron being not over 15 per cent, and the amount of manganese being about 0.5 to 2 per cent, and from 0.08 to 0.22 per cent of carbon, the composition of the ranges being so correlated that the remainder is not exceeding 0.50 per cent of elements concurrent with the aforesaid elements in commercial purities thereof and elements residual from deoxidation of the melted alloy, the said alloy having 'a'hardness of from 200 to 300 Vickers in solutionannealed condition and being capable in solutionannealed condition of reduction by cold-rolling by at least per cent. v

3. A watch mainspring characterized in being non-magnetic, non-magnetizable and non-corrodible by atmospheric conditions and body fluids, said mainspring comprising a metallic ribbon having been solution-annealed, cold-rolled and age-hardened in sequence, and so conditioned having a hardness above 600 Vickers, a proportional limit of at least 230,000 p. s. i., a yield strength (0.02 per cent offset) of at least 270,000 p. s. i., and a modulus of elasticity of at least 28,400,000 p. s. i., said ribbon being of an alloy consisting essentially of 40 per cent cobalt, 27 per cent chromium and molybdenum together, and of which 20 per cent is chromium and 7 per cent is molybdenum, 15.5 per cent of nickel, 15 per cent of iron, 2 per cent of manganese,

55 and from 0.08 to 0.22 per cent of carbon, with the remainder not exceeding 0.50 per cent of elements concurrent with the aforesaid elements in commercial purities thereof and elements residual from deoxidation of the melted alloy, the said alloy having a hardness below 300 Vickers in solution-annealed codition and being capable in solution-annealed condition of reduction by cold-rolling by at least per cent.

4. A watch mainspring characterized in being non-magnetic, non-magnetizable and non-corrodible by atmospheric conditions and body fluids, said mainspring comprising a metallic ribbon having been solution-annealed, cold-rolled and 7 age-hardened in sequence, and so conditioned 75 28,400,000 p. s. i., said ribbon being ofan alloy consisting essentially of 40 per cent cobalt, 27 per cent chromium and molybdenum together, and of which 20 per cent is chromium and 7 per cent is molybdenum, 15.5 per cent of nickel, 15 per cent of iron, 2 per cent of manganese, and from 0.08 to 0.22 per cent of carbon, with the remainder not exceeding 0.50 per cent of elements concurrent with the aforesaid elements in commercial purities thereof and elements residual from deoxidation of the melted alloy, such remainder including in addition 0.01 to 0.09 per cent of beryllium, the said alloy having a hardness below 300 Vickers in-solution-annealed condition and being capable in solution-annealed condition of reduction by cold-rolling by at least 90 per cent.

OSCAR E. HARDER. DINIQN A. ROBERTS.

REFERENCES CITED 12 UNITED STATES PATENTS Number Name Date 1,698,935 Chesterfleld Jan. 15, 1929 1,917,723 Koster July 11, 1933 1,942,150 Rohn June 2, 1934 1,949,313 Koster Feb. 27, 1934 1,974,695 Btraumann Sept. 25, 1934 2,044,165 Halliwell June 16, 1936 2,103,500 Touceda Dec. 28, '1937 2,245,366 Rohn June 10, 1941 2,419,825 Dinerstein Apr. 29, 1947 2,469,718 Edlund et a1 May 10, 1949 2,475,642 Scott et al July 12, 1949 FOREIGN PATENTS Number Country Date 510,154 Great Britain July 24, 1939 OTHER REFERENCES Age Hardening of Metals, 1940, p. 321; pub. by American Society for Metals, Cleveland, Ohio.

Progress Report on Heat Resisting Metals for Gas Turbine Parts (N-102), pub. by War Metallurgy Division N. D. R. 0., September 21, 1943, pp. 1, 6, 9, '10, 11. 

